Use extra_free_kbytes.sh whenever sys.sysctl.extra_free_kbytes changes

sys.sysctl.extra_free_kbytes property controls the value set to the
/proc/sys/vm/extra_free_kbytes out-of-tree kernel knob. Replace its
use with execution of extra_free_kbytes.sh script which calculates
and sets corresponding value to /proc/sys/vm/watermark_scale_factor
upstream-supported kernel knob.

Bug: 109664768
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Iaece28eb858a20c8c39578a28dc6bbc6adc13c40